Depth control system for marine seismic surveying



y 18, 1967 (5. a. LOPER ETAL 3,

DEPTH CONTROL SYSTEM FOR MARINE SEISMIC SURVEYING Original Filed June17, 1963 d "l BRIDGE CONTROL E AIR Q g NETWORK VALVES COMPRESSOR: l .l

FIG. 3

FIG.4A FIG. 4B

INVENTORS.

GEORGE B. LOPER JULIUS PODHRASKY,JR\

MAZW

ATTORNEY United States Patent Gflfice 3,332,058 Patented July 18, 19673,332,058 DEPTH CONTROL SYSTEM FOR MARINE SEllSMIC SURVEYING George ll.Loper and Julius Podhrasliy, Jr., Dallas, Tex., assignors to Mobil OilCorporation, a corporation of New York Continuation of application Ser.No. 288,180, June 17,

1963. This application Nov. 25, 1966, Ser. No. 597,173 9 Claims. (Cl.340-7) This application is a continuation of application Ser. No.288,180 filed on June 17, 1963, and now abandoned.

This invention relates to seismic surveying of underwater geologicformations and more particularly to the reduction of unwanted noise inseismic surveying and has for an object the provision of an improvedsystem for maintaining a seismic spread at a constant depth duringtowing operations to reduce noise thereby obtaining improved results incontinuous seismic operations.

In present day marine seismic operations, continuous seismograms areproduced by repetitively producing seismic disturbances in water from amoving vessel at short time periods, for example, every six seconds. Tocarry out these operations, the detector system of a seismic spreadtowed behind the vessel continuously detects reflected signals duringthe towing operations. In conventional towing systems, the front end ofthe spread is supported below the surface by fixedly and directlycoupling the front of the spread to a suitable float which is towedbehind the towing vessel. The difficulty with such a system is that waveaction imparts an up and down motion to the float which in turn istransmitted directly to the seismic spread thereby creating unwantednoise.

In accordance with the present invention, a system is provided fortowing a seismic spread at a constant and desired depth without the useof a surface float thereby reducing noise. More particularly, the systemincludes an inclosed container or flotation chamber for maintaining aseismic spread at a constant depth below the surface of water. Thecontainer is adapted to be towed below the surface of water by a vesseland has at least one inlet and outlet port for water. Conduit means isprovided which has a first end coupled to the interior of the containera second end extending to the surface. At the surface, means is providedfor supplying gas to the com tainer. In addition, control means iscoupled to the conduit for controlling the flow of gas into and from thecontainer to control the amount of water therein for varying thebuoyancy of the container to maintain the container at a desired depth.

In a more particular aspect, the second end of the conduit extends tothe tow vessel where the control means is coupled to a means for pumpingair into and from the container. The depth of the container isautomatically adjusted by the provision of a pressure-sensitive meanswhich produces an output of amplitude and phase indicative of deviationfrom a given depth, which means is coupled to the container and to thecontrol means. The

.control means is responsive to the phase and amplitude constant depth;

FIGURE 2 illustrates, in block diagram, 21 control system employed inthe system of FIGURE 1;

FIGURE 3 illustrates the circuitry of the control system of FIGURES 1and 2; and

FIGURES 4A and 4B illustrate valve positions of the control system.

Referring now to FIGURE 1, there will be described the system of thepresent invention for maintaining a seismic spread at a constant depth.More particularly, the system includes an enclosed container 10 coupledto a seismic spread 11 and to a tow vessel 12 by way of cable 13. Aninlet and outlet port 14 for water is located in the bottom of thecontainer 10. The buoyancy of the container is varied by injecting andwithdrawing gas from the container to control the amount of watertherein. This is accomplished by the provision of a flexible conduit 15leading from the interior of the container to system 16 located on thevessel 12. The system 16 includes means for supplying gas to thecontainer by way of conduit 15. The container 10 is automaticallymaintained at a constant depth by the use of a pressure-sensitive means17 located in the container 10 and which automatically controls the flowof gas into and from the container.

More particularly, in a preferred embodiment, the depth orpressure-sensitive means 17 comprises a pressure-sensitive transducerwhich includes a resistive ele- -ment 18, the resistance of which variesin proportion to and from the container 10 by way of conduit 15. An

advantage of such a system is that it is simple yet rugged in that nomoving parts are employed on the container which otherwise would besubject to the corrosive actions of the water. Furthermore, thetransducer employed requires very little current, for example, a fewmilliamps. Thus, mechanical and electrical noise is reduced to a minimumwhich otherwise would affect the seismic record. In addition, anunlimited supply of air is provided by way of conduit 15 for controllingthe depth of the container 10.

Referring now to FIGURES 3 and 4, there will be described the operationof the bridge network 19 for con trolling the valve arrangement 20 whichcomprises two three-way valves 22 and 23 coupled to air compressor 21.The bridge comprises the resistive element 18 connected so that theresistance thereof increases as the depth and hence as the pressureincreases and conversely decreases as the depth increases. Coupled toresistive element 18 is a two-stage amplifier 24, the output of which iscoupled to two valve actuating coils 25 and 26. These coils control themovement of valve elements 27 and 28 respectively of valves 22 and 23 tocontrol the flow of air into and from conduit 15. As the depth ofcontainer 10 is to be maintained. Coupled across resistive element 18and resistor 29 and across fixed and equal resistors 31 and 32 is an A-Cpower supply 33. The amplifier 24 is coupled at the juncture of element18 and resistor 29' and at the juncture of resistors 31 and32,respectively, at points a and b. The output of the second stage ofamplifier 24 is coupled to the primary of transformer 34. The secondaryof this transformer is center tapped at 35 and coupled at the junctureof the two valve coils 25 and 26. Also coupled to power supply 33 istransformer 36, the secondary of which is coupled to center tap 35 andat the juncture of the valve coils 25 and 26. Briefly, when thecontainer is at the desired depth as set by resistor 29, the resistanceof element 18 is equal to that of resistor 29. The voltage at point a isequal to that at point [2 and zero output is applied to amplifier 24.When the depth deviates from the desired depth, however, the resistanceof element 18 changes and an error output is applied to amplifier 24.The error output is amplified and applied to actuate either of coils 25or 26 to allow air to flow into or from container 10. The operation ofthe bridge will become apparent from the following examples now to begiven.

When the container is at the surface, air is injected into the containerto keep the water out as will be described hereinafter. When it isdesired to position contained 10 at a desired depth, resistor 29 isadjusted to a predetermined value determined from prior calibration, forexample, to 2,500 ohms. The resistances of resistors 31 and 32, forexample, each may be of the order of 2,500 ohms. At the surface, theresistance of element 18 is smaller than 2,500 ohms. When point e ispositive with respect to point 1 the voltage at point a is more positivethan the voltage at point b. Thus, the grid of the first stage ofamplifier 24 is negative with respect to the cathode. This, in turn,produces a negative output from the anode of the second stage ofamplifier 24. From the dot convention shown, current will flow in thesecondary windings of transformer 34 in the direction of arrows c and d.At this instant, as the output from A-C power supply 33 becomes positiveat point e with respect to point 1, from the dot convention shown,current will flow in the secondary of the transformer 36 in thedirection of arrows g and h. Thus, the currents illustrated at c and gare additive, thereby causing valve coil 25 to become energized.Currents illustrated at d and h flow in opposite directions to causecoil 26 to become deenergized. When this occurs, control element 28 ofvalve 23 is moved by biasing spring 37 to the position illustrated toallow compressor 21 to with-draw air from container 10. When coil 25 isenergized, mechanical connections 38 overcomes the bias of spring 39 andmoves control element 27 of valve 22 to the position illustrated toprevent air from fiowing into conduit 15. When air is withdrawn fromcontainer 10, water enters the container by way of port 14, therebycausing the container 10 to sink. As the container sinks, the resistanceof element 18 increases due to the increase in pressure. The container10 Will continue to sink until the resistance of element 18 equals theresistance of resistor 29. When this occurs, there is no current flow inthe primary hence in the secondary of the transformer 34. Thus, ,equalamounts of current from the secondary of transformer 36 flow through thecoils 25 and 26 thereby causing both coils to become energized. In theenergized condition, mechanical connections 38 and 40 move controlelements 27 and 28 to a position wherein air is neither withdrawn norinjected into the conduit 15, thereby allowing the container 10 toremain at the desired depth. FIGURE 4A illustrates the position ofcontrol elements 27 and 28 when coils 25 and 26 are both energized.

If, for some reason, the depth of the container 10 sinks below thedesired depth, due, for example, to a change in speed of the tow vessel12, the resistance of element 18 will increase, thereby becoming greaterthan the resistance 29. When 2 is positive with respect to f, thevoltage at point a will be negative with respect to point b, and thevoltage applied to the grid of the first stage of amplifier 24- will bepositive with respect to the cathode. When this occurs, the current fiowin the secondary of transformer 34 will be in a direction opposite thedirection of arrows c and d. Thus, valve coil 25 will becomede-energized and valve coil 26 will be energized. When this occurs,control elements 27 and 28 will be moved to the positions illustrated inFIGURE 4B to allow air to flow into conduit 15, thereby forcing waterout of container 10 to raise the level of the container. As thecontainer 10 rises, the resistance of element 18 will decrease until itreaches that of resistor 29 wherein coils 25 and 26 both will becomeenergized to stop the flow of air into conduit 15 to maintain thecontainer 10 at the desired depth.

When the vessel 12 initially tows the spread to the desired seismicsurveying position, air is injected into container 10 to maintain thecontainer and spread at the surface. Switch 41 is provided to carry outthis function. When the switch is closed, resistor 29 is shortcircuited. As now can be understood, the input to the grid of the firststage of amplifier 24 is positive with respect to the cathode and theoutput of amplifier 24 causes valves '22 and 23 to be actuated to allowair to flow into container 10.

Referring again to FIGURE 1, it can be seen that the leads of resistiveelement 18 extends to the control system 16 by way of the cable 13. Theoutput of seismic spread 11 is applied to conductor 42, which alsoextends through the cable 13 to a recording system (not shown) on thevessel 12. The seismic spread 11 is a conventional spread, which isneutrally buoyant in that it barely floats in water and is thusmaintained at the depth of container 10. Resilient springs 43 and theweight of the container 10 together act to absorb jerking motionsimparted by the towing operations to reduce noise further. A weightedkeel 44 is provided to maintain the container 10 in an upright position.

In one embodiment, the transducer 18 was a type 401 pressure transducer,produced by Colvin Laboratories, Inc., East Orange, NJ. The transducerincluded a 5000 ohm potentiometer resistance. The power supply 33 was a24-vo1t, 60-cycles per second power supply. The valves 22 and 23 were aMarsh Type 36 three-way solenoid valve, direct acting and spring loaded,produced by Marsh Instrument Company, Skokie, Ill.

While the invention has been described in connection with a specificembodiment thereof, it will be understood now that further modificationswill suggest themselves to those skilled in the art and it is intendedto cover such modifications as fall within the scope of the appendedclaims.

We claim:

1. A system for controlling the depth of a marine seismic detector cablewhich is towed behind a vessel, comprising:

a flotation chamber for attachment to said detector cable, saidflotation chamber having means for admitting and discharging water intoand out of said chamber,

a source of compressed gas for mounting on said vessel,

conduit means for coupling the interior of said flotation chamber tosaid source of compressed gas, and

control means for mounting on said vessel and for controlling theinjection of said gas via said conduit means into said flotation chamberto discharge the water therein and increase the buoyancy thereof, thuscausing said flotation chamber and said detector cable to rise, saidcontrol means being operable to withdraw the gas from said flotationchamber via said conduit means, thus permitting admittance of watertherein to cause decrease in buoyancy thereof and consequent lowering ofsaid flotation chamber and said detector cable.

2. The system of claim 1 further comprising:

a depth-sensitive means for producing an output signal representative ofthe depth of said detector cable, and

valve means for mounting on said vessel and for adjusting said controlmeans in accordance with said output signal.

3. A system for maintaining at a predetermined constant depth, a marineseismic detector cable which is towed behind a vessel in a body ofwater, said system comprising:

an elongated container for attachment to said detector cable and adaptedto be towed underwater, the outer wall of said container forming aninterior flotation chamber, said container having an air port in thefront portion thereof, said container having a water port which, duringoperation of the system, is at a level below that of said air port, saidwater port being adapted for transferring water from the body of waterinto and out of said flotation chamber,

a source of compressed air for mounting on said vessel,

valve means, for mounting on said vessel and for coupling to said sourceof compressed air. a conduit for coupling said air port of saidcontainer to said valve means,

depth-sensitive means for producing an output signal representative ofthe depth of said detector cable, and,

said valve means being adjustable according to said output signal .tocontrol the injection of said gas via said conduit into said flotationchamber to discharge the water therein through said water port andincrease the buoyancy of said container, thus causing said container andsaid detector cable to rise, said valve means being operable to withdrawvia said conduit the gas in said flotation chamber, thus permitting theadmittance of water therein through said water port to decrease thebuoyancy of said container and consequent lowering of said container andsaid detector cable.

4. A depth-control system for marine seismic surveying comprising anenclosed container for maintaining a seismic spread at a constant anddesired depth below the surface of water, said container being adaptedto be towed below the surface of water by a vessel and having at leastone inlet and outlet port for water, conduit means having a first endcoupled to the interior of said container and a second end extending tothe surface, and means coupled to said second end for pumping air intoand from said container by way of said conduit for varying the buoyancyof said container to maintain said container at said desired depth.

5. A depth-control system for marine seismic surveying comprising anenclosed container for maintaining a seismic spread at a constant anddesired depth below the surface of the water, said container beingadapted to be towed below the surface of the water by a vessel andhaving at least one inlet and outlet port for water, conduit meanshaving a first end coupled to the interior of said container and asecond end extending to the surface, depth-sensitive means locatedwithin said container for producing output signals in response tovariations in the depth of said container, pump means disposed on thesurface for pumping gas into and out of said container by way of saidconduit, and control means responsive to said output signals and coupledto said second end of said conduit and said pump means for selecting andregulating the flow of gas pumped into and from said container to thusvary the buoyancy thereof in maintaining said container at said desireddepth.

6. A depth-control system for marine seismic surveying comprising anenclosed container for maintaining a seismic spread at a constant anddesired depth below the surface of the water, said container beingadapted to be towed below the surface of the water by a vessel andhaving at least one inlet and outlet port for water, a conduit having afirst end coupled to the interior of said first container and second endextending to the surface, depthsensitive means located within saidcontainer for produc ing output signals in response to variations in thedepth of said container, pump means disposed on the surface, and controlmeans responsive to said output signals and coupled to said pump meansand said second end of said conduit for selectively connecting said pumpmeans with said conduit to cause gas to be pumped into or from saidcontainer to vary the buoyancy thereof in maintaining said container atthe desired depth.

7. The depth-control system of claim 6 wherein said control means closessaid second end of said conduit when said container reaches the desireddepth to prevent further delivery of air to or from said container.

8. A depth-control system for marine seismic surveying comprising anenclosed container for maintaining a seismic spread at a constant anddesired depth below the surface of the water, a conduit connected tosaid container and extending to the surface, gas pump means havingintake and discharge ports, an alternating current source, meansconnected to said alternating current source for generating an outputfunction, the phase and amplitude of which is indicative of thedirection and extent of deviation of said container from a predetermineddepth,

and valve control means responsive to the phase comparison of saidoutput function with said alternating current source to close saidconduit when said output function is of zero amplitude and selectivelyopen said conduit to said intake and discharge ports of said gas pump inaccordance with said phase comparison to cause gas to be pumped to orfrom said container and vary the buoyancy thereof in accordance with thecontainers deviation from the desired depth.

9. The depth-control system of claim 8 in which said valve control meansincludes two solenoid actuated valves connected to said intake anddischarge ports of said gas pump means and having their respectiveoperating coils directly in circuit one with another and inductivelycoupled in series opposition relationship with said alternatng currentsource and in series aiding relationship with said output function, saidvalves assuming the same valve position upon said output functionreaching zero and assuming opposite valve positions dependent upon thephase comparison of said reference source with said output function whensaid output function is other than zero.

No references cited.

SAMUEL FEINB-ERG, Primary Examiner. P. A. SHANLEY, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,332,058 July 18, 1957 George B. Loper et a1 It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 1 line 44 after "container" insert and column 2 line 48 for"increases" read decreases column 3 lines 17 and 18 for "contained" readcontainer lines 42 and 43 for "connections" read connection line 52after "primary" insert and column 4, line 20 for "extends" read extendcolumn 5 line 15 for "means read means line 16 for "air." read air,column 6 lines 50 and 51 for "alternatng" read alternating Signed andsealed this 18th day of June 1968 (SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

1. A SYSTEM FOR CONTROLLING FOR THE DEPTH OF A MARINE SEISMIC DETECTORCABLE WHICH IS TOWED BEHIND A VESSEL, COMPRISING: A FLOTATION CHAMBERFOR ATTACHMENT TO SAID DETECTOR CABLE, SAID FLOTATION CHAMBER HAVINGMEANS FOR ADMITTING AND DISHCARGING WATER INTO AND OUT OF SAID CHAMBER,A SOURCE OF COMPRESSED GAS FOR MOUNTING ON SAID VESSEL, CONDUIT MEANSFOR COUPLING THE INTERIOR OF SAID FLOTATION CHAMBER TO SAID SOURCE OFCOMPRESSED GAS, AND CONTROL MEANS FOR MOUNTING ON SAID VESSEL AND FORCONTROLLING THE INJECTION OF SAID GAS VIA SAID CONDUIT MEANS INTO SAIDFLOTATION CHAMBER TO DISCHARGE THE WATER THEREIN AND INCREASE THEBUOYANCY THEREOF, THUS CAUSING SAID FLOTATION CHAMBER AND SAID DE-